4936 molecular isomerization of a molecule in a thermal system. In the latter case, with increasing complexity of the molecule, at any given pressure, the rate of unimolecular isomerization will correspond more nearly to its value at infinite pressure. In the present case, since the system involves the stabilization of a ‘‘hot” molecule, an increase in the complexity of the molecule at a given pressure reduces its isomerization (to a cyclopropyl compound) rate more nearly to zero which would be its value at infinite pressure. Thus, both 2-methyl1,4-pentadiene and 1,5-hexadiene, which are isomers, give the same yield of cyclopropyl products. Both 2-methyl- 1,5-hexadiene and 1,6-heptadiene, which form another isomeric pair, give very little cyclopropyl products at 1 atm, although at lower pressures they do isomerize to such compounds to a significant extent.I4s15 Perhaps the most interesting result of this work is the change in the stereochemistry of the cycloaddition reaction as the chain length increases. Cross addition to parallel addition is seen to be highly selective in the pentadienes, a feature which is more emphasized by methyl substitution. In the hexadienes the importance of the two modes inverts itself and becomes less selective, although in 1,5-cyclooctadiene the process is very (15) R. Srinivasan, unpublished work.
selective. In 1,6-heptadiene the process inverts its importance once again and is also quite selective, These results can be explained by assuming that the cycloaddition is a two-step process and that the initial step is the formation of a five-membered ring. These reactions can be written as follows.
< e-
-
4
+
(14)
(15)
The formation of the alternative cycloadducts cannot go through similar five-membered rings. This hypothesis can be tested by the examination of other systems which incorporate these dienes as well as by placing substituent(s) at the unsaturated carbons to see if the intermediates in steps 13-15 allow free rotation at the radical sites. Such investigations are now in progress. Acknowledgment. The assistance of Mr. Fred W. Hetzel of Ohio State University in recording the nmr spectra is gratefully acknowledged.
Photosensitized Internal Addition of Dienes to Olefins Robert S . H. Lid and George S. Hammond
Contribution No. 1331 f r o m the Central Research Department, Experimental Station, E. I. du Pont de Nemours and Company, Wilmington, Delaware 19898, and Contribution No. 3481 from The Gates and Crellin Laboratories of Chemistry, California Institute of Technology, Pasadena, California. Received M a y 4, 1967 Abstract: When irradiated in the presence of photosensitizers, myrcene cyclizes to give 5,5-dimethyl-l-vinylbicyclo[2.1,1]hexane (1). 3-Methylene-l,5-hexadiene, 3-methylene-l,6-heptadiene,and 6-methyl-3-methylene1,5-heptadiene cyclize under similar conditions to the corresponding bicyclo[2.1 .l]hexanes. The geometric isomers of 3-methylene-1,5-heptadienegive identical mixtures of 5-methyl-2-methylenebicyclo[2.1.l]hexanes. The result indicates that cyclization involves formation of long-lived diradical intermediates. Preferential cyclization by way of intermediates containing five-membered rings is noted and discussed.
C
onjugated dienes dimerize efficiently when irradiated in the presence of photosensitizers. Cross-additions between dienes under similar conditions have also been r e p ~ r t e d . ~ Scattered ,~ reports have also appeared on the intermolecular cycloaddition of diene triplets to Although the reactions often lead to fairly complex product mixtures, some selectivity is normally observed. For example, in the dimerization 2s
(1) To whom inquiries should be addressed, E. I. du Pont de Nemours and Co., Wilmington, Del. (2) G. S . Hammond, N. J. Turro, and R. S. H. Liu, J . Org.Chem., 28, 3297 (1963).
( 3 ) D . Valentine, N. J. Turro, and G. S. Hammond, J. Am. Chem. SOC.,86, 5202 (1964). (4) G . Sartori. V. Turba, A. Valvasson, and M. Riva, Tetrahedron Letrers, 211, 4777 (1966). ( 5 ) R. S . H. Liu, C. D. DeBoer, and G. S . Hammond, manuscript
of butadiene, 1,3-disubstituted cyclobutanes were not observed.2 Examination of the product structures has led to the hypothesis that exclusion of certain products is a result of selective formation of diradical intermediates. * A * There is also a commonly held opinion that addition of a triplet to a singlet must involve formation of an intermediateag This premise is usually known as “the Wigner spin-conservation rule” and states that processes involving changes in the multiplicity of systems will be relatively slow. Application to addition reactions of the type under discussion leads to formulation of the spin inversion that must occur at some stage as a
in preparation. (6) N. J. Turro and P. D. Bartlett, J . Org. Chem., 30, 1849 (1965). (7) R. S . H. Liu and G. S . Hammond, J . Am. Chem. SOC.,86, 1892
(8) G. S . Hammond and N. J. Turro, Science, 142, 1547 (1963). (9) See, for example, P. S . Skell and R. C. Woodworth, J . Am. Chem. SOC.,18, 4496 (1956). (10) I
